Laser scanning unit and method and apparatus for calibrating a post-scan assembly in a laser scanning unit

ABSTRACT

A laser scanning unit is provided comprising a housing; a scanning assembly including a scanning device; a first light beam source directing a first light beam toward the scanning device; a second light beam source directing a second light beam toward the scanning device; and first and second post-scan optical assemblies located to receive first and second scanning beams reflected from the scanning device. Each post-scan optical assembly may comprise a first, second and third fold mirror, a first f-theta lens located between the first and the second fold mirrors, and a second f-theta lens located to receive light reflected from the third fold mirror and outputting a compensated scanning beam for generating a scan line on a corresponding photoconductive member.

RELATED APPLICATION

[0001] This application is related to commonly assigned U.S. patentapplication Ser. No. ______, entitled “A COLLIMATION ASSEMBLY ANDMETHODS AND APPARATUS FOR CALIBRATING COLLIMATION AND PRE-SCANASSEMBLIES IN A LASER SCANNING UNIT”; filed concurrently herewith;having Attorney Docket No. 2002-0214.03; the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION

[0002] The present invention relates to laser scanning devices and, moreparticularly, to a laser scanning unit and methods for aligning theoptical elements of a laser scanning unit.

BACKGROUND OF THE INVENTION

[0003] Optical systems used in laser printers may be characterized ashaving three sub-systems or assemblies, namely, a laser diode/pre-scanoptical assembly, a scanning assembly, and a post-scan assembly.Typically, the laser diode/pre-scan optical assembly includes a laserdiode emitting a diverging laser beam, a collimator lens for collimatingthe beam emitted by the laser diode, and a pre-scan lens to focus thebeam to a waist near the scanning device so that the post-scan assemblyimages the beam to a waist at a corresponding photoconductive (PC) drumsurface.

[0004] The scanning assembly generally includes a scanning device suchas a motor driven, rotatable polygon mirror having a plurality ofperipheral mirror surfaces or facets that rotate during operation of theprinter. The mirror surfaces reflect the collimated and focused beamreceived from the laser diode/pre-scan optical assembly. The directionof rotation of the polygon mirror determines the scan direction of thebeam passing along a scanned object, such as a PC drum in a laserprinter.

[0005] U.S. Pat. No. 4,578,688 to Okuno discloses a printer having twolaser diodes emitting two laser beams reflected off of a common polygonmirror. The '688 patent teaches reducing registration errors by havingthe two laser beams contact the polygon mirror such that “the directionsof inclination of the beams La and Lb relative to the imaginary planeperpendicular to the rotary shaft 10 of the polygon mirror 3 areopposite to each other,” see column 4, lines 27-30. Hence, “thedirections in which the scanning lines Aa and Ab are curved are thesame,” see column 4, lines 30 and 31.

[0006] There is a need for a laser scanning unit comprising at least twolaser diodes for emitting a plurality of laser beams wherein thecorresponding post-scan optical assemblies are configured so that thespace on either side of the scanning assembly is efficiently used so asto facilitate minimization of the space requirements of the laserscanning unit. There also is a need for an optical system for a laserscanning unit in which the optical components may be readily alignedand, in particular, a system and a method of alignment in whichpost-scan optics may be used to readily adjust the process position andbow of scan lines.

SUMMARY OF THE INVENTION

[0007] In accordance with a first aspect of the present invention, alaser scanning unit is provided comprising a housing; a scanningassembly including a scanning device; a first light beam sourcedirecting a first light beam toward the scanning device; a second lightbeam source directing a second light beam toward the scanning device;and first and second post-scan optical assemblies located to receivefirst and second scanning beams reflected from the scanning device. Eachpost-scan optical assembly comprises a first, second and third foldmirror, a first f-theta lens located between the first and the secondfold mirrors, and a second f-theta lens located to receive lightreflected from the third fold mirror and output a compensated scanningbeam for generating a scan line on a corresponding photoconductivemember.

[0008] Light reflected from each of the second fold mirrors toward arespective one of the third fold mirrors may extend in a plane which isgenerally parallel to a plane containing the light beam produced by arespective one of the first and second light beam sources.

[0009] Preferably, at least one of the first and second post-scanoptical assemblies includes an adjustment mechanism for adjusting itscorresponding second f-theta lens whereby a scan line formed by ascanning beam passing through the corresponding second f-theta lens isadjusted. The corresponding second f-theta lens includes opposing, firstand second longitudinal ends and the adjustment mechanism may provideindependent adjustment of the first and second ends to adjust one ormore of a process direction position of the scan line, bow of the scanline, and skew orientation of the scan line.

[0010] Preferably, the third fold mirror of at least one of the firstand second post-scan optical assemblies is rotatably adjustable about alongitudinal axis of the third fold mirror to adjust the bow and processdirection position of the corresponding scan line. It is also preferredthat each of the first fold mirrors reflects a respective scanning beaminwardly and downwardly. The second fold mirrors of the first and secondpost-scan optical assemblies may be located substantially below thescanning device for receiving the reflected scanning beams passingthrough the first f-theta lenses from the first fold mirrors.

[0011] The scanning device may comprise a polygon mirror supported forrotation about a rotational axis. The first light source and the secondlight source are preferably located on the same side of a planecontaining the rotational axis of the polygon mirror and extendingperpendicular to the scan lines. The first and second light sources arealso preferably located at substantially similar mirror image angles toparallel lines which are substantially parallel to the scan lines.

[0012] In accordance with a second aspect of the present invention, alaser scanning unit is provided comprising: a scanning assemblyincluding a scanning device; at least one light beam source directing alight beam toward the scanning device; at least one post-scan opticalassembly including a plurality of optical components; an upper housingportion and a lower housing portion wherein the upper and lower housingportions are joined together to form a housing for supporting thescanning assembly, the light beam source and the post-scan opticalassembly. Preferably, at least one of the optical components is mountedto the upper housing portion and at least one of the optical componentsis mounted to the lower housing portion.

[0013] The at least one optical component mounted to the lower housingportion may be adjustable to position a scan line generated by ascanning beam output by the post-scan optical assembly.

[0014] The post-scan optical assembly may comprise a first, second andthird fold mirror, a first f-theta lens located between the first andthe second fold mirrors, and a second f-theta lens located to receivelight reflected from the third fold mirror. The first and second foldmirrors and the first f-theta lens may be mounted to the upper housingportion, and the third fold mirror and the second f-theta lens may bemounted to the lower housing portion. The upper housing portion includesupper and lower sides or surfaces and the first fold mirror may bemounted to the upper side and the second fold mirror and the firstf-theta lens may be mounted to the lower side.

[0015] The laser scanning unit may comprise two light beam sources andfirst and second post-scan optical assemblies located to receive firstand second scanning beams reflected from the scanning device. Thepost-scan optical assemblies each comprising a plurality of opticalcomponents and wherein at least one of the optical components of each ofthe first and second post-scan assemblies is mounted to the upperhousing portion and at least one of the optical components of each ofthe first and second post-scan assemblies is mounted to the lowerhousing portion.

[0016] The upper and lower housing portions may include first and secondalignment structures. The first alignment structure may comprise a pinin one of the housing portions engaging a hole in the other of thehousing portions, and the second alignment structure may comprise a pinin the one of the housing portions engaging an elongated slot in theother of the housing portions, whereby the upper and lower housingportions are aligned with each other.

[0017] In accordance with a third aspect of the present invention, amethod is provided for calibrating a laser scanning unit comprising ahousing, a first light beam source, a scanning assembly including ascanning device, and a first post-scan assembly. The first post-scanassembly includes at least one fold mirror, and at least one f-thetalens located downstream of the at least one fold mirror to receive lightreflected by the fold mirror and output a compensated first scanningbeam for generating a first scan line on a first photoconductive member.The first scan line extends in a scan direction which is substantiallyperpendicular to a process direction. The method comprises the steps of:installing the f-theta lens at a nominal location and, thereafter,moving the fold mirror to change the first scan line to obtain a firstbow value within a predetermined range and, thereafter, moving thef-theta lens to move the first scan line in the process direction toobtain at least one of a second bow value and desired process locationfor the first scan line.

[0018] The method may further include the step of moving the f-thetalens to adjust a skew orientation of the scan line relative to theprocess direction.

[0019] The first post-scan assembly may comprise a first, second andthird fold mirror, a first f-theta lens located between the first andsecond fold mirrors, and a second f-theta lens located to receive lightreflected from the third fold mirror and outputting the first scanningbeam for generating the first scan line. The step of moving the foldmirror preferably includes moving the third fold mirror to change thefirst scan line to obtain a first bow value within a predetermined rangeand the step of moving the f-theta lens preferably includes moving thesecond f-theta lens to move the first scan line in the process directionto obtain a second bow value and desired process location for the firstscan line.

[0020] The step of moving the f-theta lens to move the first scan linein the process direction preferably results in the scan line beingadjusted to a final, desired bow value and a final, desired processlocation.

[0021] In accordance with a fourth aspect of the present invention, alaser scanning unit is provided comprising: a scanning assemblyincluding a scanning device; a light beam source directing a light beamtoward the scanning device; a post-scan optical assembly including apivotal mirror and a movable f-theta lens. The f-theta lens ispositioned to receive light reflected by the mirror and output acompensated scanning beam for generating a scan line on a correspondingphotoconductive member. The mirror and the f-theta lens are adjustableto move the position and correct bow of the scan line.

[0022] The f-theta lens is preferably longitudinally movable to correctbow of the scan line and to move the position of the scan line in aprocess direction perpendicular to a scan direction of the scanning beamand is pivotally movable to correct a skew orientation of the scan line.

[0023] The pivotal mirror preferably defines a third mirror and thef-theta lens preferably defines a second f-theta lens and the post-scanoptical assembly further comprises a first and a second fold mirror, anda first f-theta lens located between the first and second fold mirrors,and wherein the second f-theta lens is located to receive lightreflected from the third fold mirror and output the compensated scanningbeam for generating the first scan line.

[0024] In accordance with a fifth aspect of the present invention, alaser scanning unit is provided comprising: a scanning assemblyincluding a scanning device; a light beam source directing a light beamtoward the scanning device; and a post-scan optical assembly includingan f-theta lens for receiving a scanning beam reflected by the scanningdevice and outputting a compensated scanning beam for generating a scanline on a corresponding photoconductive member. The f-theta lens ispreferably movable to correct a skew orientation of the scan line.

[0025] The f-theta lens may be supported for pivotal movement to correctthe skew orientation.

[0026] The f-theta lens may define a second f-theta lens and thepost-scan optical assembly further comprises a first, second and thirdfold mirror, and a first f-theta lens located between the first andsecond fold mirrors. The second f-theta lens is preferably located toreceive light reflected from the third fold mirror and output thecompensated scanning beam for generating the first scan line.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a perspective view of the scanning assembly and firstand second post-scan optical assemblies removed from a housing of alaser scanning unit of the present invention;

[0028]FIG. 2 is a perspective, top view of a laser scanning unit of thepresent invention;

[0029]FIG. 3 is a view of a second or lower side of an upper housingportion of the laser scanning unit of FIG. 2;

[0030]FIG. 4 is perspective view of an upper surface or side of a lowerhousing portion of the laser scanning unit of FIG. 2;

[0031]FIG. 5 is a view, partially in cross section, illustrating anadjusting screw for a third fold mirror and an adjusting screw for asecond f-theta lens;

[0032]FIG. 6 is a perspective view of a lower surface or side of thelower housing portion of the laser scanning unit of FIG. 2;

[0033]FIG. 7 is a perspective view of a post-scan optical assemblyremoved from a scanning unit housing and illustrating camera units of acalibration arrangement for use in adjusting bow and process position ofa scan line generated by a compensated scanning beam output by thepost-scan optical assembly;

[0034]FIG. 7A is a view of a post-scan optical assembly removed from ascanning unit housing and illustrating locations of specific pointsalong an optical path defined by the components of the post-scanassembly;

[0035]FIG. 7B is a cross-sectional view of a first f-theta lens;

[0036]FIG. 7C is a cross-sectional view of a second f-theta lens;

[0037]FIG. 8 is a perspective view of a laser scanning unit withcomponents not yet assembled thereto and illustrating a quad cell sensorfor use in aligning collimation assemblies provided on the scanning unithousing;

[0038]FIG. 9 is an enlarged perspective view of a collimation assemblyof the laser scanning unit of FIG. 2; and

[0039]FIG. 10 is a perspective view of a post-scan optical assemblyremoved from a scanning unit housing and illustrating a camera unit of acalibration arrangement for use in adjusting the position of a pre-scanlens and its carrier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] With reference to FIGS. 1 and 2, an optical system 5 for a laserscanning unit 7 of the present invention is illustrated, and includes afirst light beam source 10, see FIG. 2, a second light beam source 12, ascanning assembly 14, and first and second post-scan optical assemblies16, 18, see FIG. 1. In the illustrated embodiment, the first light beamsource 10 comprises a first laser diode/pre-scan optical assembly 11 andthe second light beam source 12 comprises a second laser diode/pre-scanoptical assembly 13. It should be noted that the first and second laserdiode/pre-scan optical assemblies 11, 13 include substantially the samecomponents and the first and second post-scan optical assemblies 16, 18include substantially the same components, and similar components of thefirst and second laser diode/pre-scan optical assemblies and post-scanoptical assemblies are labeled with similar numbers having suffixes “a”and “b” to distinguish between the components of correspondingassemblies.

[0041] The first and second post-scan optical assemblies 16, 18 outputcompensated scanning beams which generate scan lines 54 a and 54 b onrespective photoconductive drums 20, 22, see FIG. 1. It is contemplatedthat two scanning units 7 may be used in combination in a color laserprinter in order to generate four latent images on four corresponding PCdrums. The four latent images are developed by developing apparatus (notillustrated) so as to generate four toner images, e.g., yellow, cyan,magenta and black toner images, which are registered to one another toform a composite toner image.

[0042] The first and second laser diode/pre-scan optical assemblies 11,13 are located on the same diametric side of the scanning assembly 14,see FIG. 2. Each assembly 11, 13 includes a laser diode/collimationassembly structure 24 a, 24 b and a pre-scan lens structure 26 a, 26 b.Each laser diode/collimation assembly structure 24 a, 24 b includes alaser driver card 28 a, 28 b mounted to a collimation housing 30 a, 30b. A collimator lens (not shown) is provided in each collimation housing30 a, 30 b. The collimation housing 30 a and corresponding collimatorlens define a first collimation assembly 27 a and the collimationhousing 30 b and corresponding collimator lens define a secondcollimation assembly 27 b. Each driver card 28 a, 28 b comprises a laserdiode (not shown) for producing a laser beam which passes through acorresponding collimator lens. The collimator lenses function tosubstantially collimate the laser beams emitted from the diodes. Itshould be noted that throughout the present description of the inventionand the appended claims, reference to a collimated laser beam andsubstantially collimated laser beam includes beams which are slightlydiverging, beams which are slightly converging, as well as beams havingparallel rays.

[0043] Each pre-scan lens structure 26 a, 26 b comprises a pre-scan lens32 a, 32 b provided in a carrier 34 a, 34 b. Further, each carrier 34 a,34 b comprises an elliptical aperture (not shown) for defining the spotsize of a corresponding laser beam.

[0044] The scanning assembly 14 includes a scanning device which, in theillustrated embodiment, comprises a rotatable polygon mirror 36. Themirror 36 has a plurality of mirror facets 38, eight facets in theillustrated embodiment, for reflecting the laser beams originating fromeach of the laser diode/pre-scan optical assemblies 11, 13. The polygonmirror 36 is driven in rotation about a rotational scanning axis 40 at asubstantially constant speed by a scanning motor 42, see FIG. 1, whereinthe beams from the laser diode/pre-scan optical assemblies 11, 13 arereflected by different facets 38 on the same diametric half of thepolygon mirror 36 to produce two scanning beams 53 a and 53 b, see FIG.1.

[0045] It should be noted that within the scope of the presentinvention, the scanning assembly may incorporate other types of scanningdevices. For example, a micro-mirror scanning device may be incorporatedinto the scanning assembly in place of the polygon mirror 36. Amicro-mirror scanning device generally includes a movable platecontaining a reflective surface supported for oscillating movement abouta pivotal scanning axis, movement of which is induced throughelectromagnetic forces. Such a micro-mirror scanning device is disclosedin U.S. patent application Ser. No. 10/093,754, filed on Mar. 8, 2002,which application is assigned to the assignee of the present inventionand is incorporated herein by reference.

[0046] Each of the first and second post-scan optical assemblies 16, 18comprises, in order along a corresponding optical path, a first foldmirror 44 a, 44 b, a first f-theta lens 46 a, 46 b, a second fold mirror48 a, 48 b, a third fold mirror 50 a, 50 b and a second f-theta lens 52a, 52 b, see FIG. 1. The two scanning beams 53 a, 53 b reflected fromthe polygon mirror 36 impinge upon the first fold mirrors 44 a, 44 b andare reflected downwardly by those fold mirrors 44 a, 44 b. Thedownwardly reflected beams 53 a, 53 b pass through a corresponding firstf-theta lens 46 a, 46 b. Thereafter, each beam is reflected in asubstantially horizontal direction by a second fold mirror 48 a, 48 b toa third fold mirror 50 a, 50 b. The beams reflected from the third foldmirrors 50 a, 50 b then pass through a corresponding second f-theta lens52 a, 52 b. The scanning beams exiting the second f-theta lenses 52 a,52 b generate a corresponding scan line 54 a, 54 b on a respective oneof two photoconductive drums 20, 22, see FIG. 1. The first and secondf-theta lenses 46 a, 46 b, 52 a, 52 b function in combination tolinearize, focus, locate and generate the spot size of the beams formingthe scan lines 54 a, 54 b.

[0047] Referring to FIGS. 2-4, the optical system 5 is mounted to ahousing 56 which comprises a two-part assembly including an upperhousing portion 58 and a main or lower housing portion 60, wherein atleast some of the optical components of the optical system 5 are mountedto each of the upper and lower housing portions 58, 60. In particular,the first and second laser diode/pre-scan optical assemblies 11 and 13,the scanning assembly 14, the first fold mirrors 44 a, 44 b, the firstf-theta lenses 46 a, 46 b and the second fold mirrors 48 a, 48 b aremounted to the upper housing portion 58, and the third fold mirrors 50a, 50 b and the second f-theta lenses 52 a, 52 b are mounted to thelower housing portion 60.

[0048] As best seen in FIG. 2, the first fold mirrors 44 a, 44 b areheld in stationary position on a first or upper side 62 of the upperhousing portion 58 by mirror supports 64 a, 64 b molded into the upperhousing portion 58. In addition, a pair of spring clips 66 a, 66 b holdthe mirrors 44 a, 44 b in position on the supports 64 a, 64 b.

[0049] Referring to FIG. 3, a second or lower side 68 of the upperhousing portion 58 is shown wherein the first f-theta lenses 46 a, 46 bare held in stationary position by a support structure 70 a, 70 b andspring clips 72 a, 72 b. The second fold mirrors 48 a, 48 b are alsomounted to the second side 68 of the upper housing portion 58 and areheld in position by support members 74 a, 74 b, and a pair of springclips 76, 78, each of the spring clips 76, 78 having a pair of opposingresilient elements 80 a, 80 b for engaging respective ends of each ofthe second fold mirrors 48 a, 48 b.

[0050] Referring to FIG. 4, the lower housing portion 60 includes agenerally planer plate portion 82, vertical end portions 84, 86extending generally perpendicular to the plate portion 82, and generallyparallel side portions 88, 90 extending perpendicular to the plateportion 82.

[0051] The third mirror 50 a is supported on a pair of mirror mounts 92a wherein the corresponding third mirror 50 b for the second scanningbeam path is not shown in order to better illustrate the mirror mounts92 b. Referring also to FIG. 5, the mirror mounts 92 a, 92 b eachinclude a respective lower mirror support 94 a, 94 b and mirror pivotmount 96 a, 96 b for providing contact points for the mirrors 50 a, 50b. A pair of spring clips 98 a (only one pair shown) are providedadjacent the ends of each mirror 50 a, 50 b for holding the mirrors 50a, 50 b in engagement with the mirror mounts 92 a, 92 b.

[0052] Each mirror 50 a, 50 b is provided with an adjusting screw 100 a(only one shown, see FIG. 5) located within a recessed portion 102 a,102 b (see also FIG. 2) of the sides 88, 90 of the lower housing portion60 and engaging a respective mirror 50 a, 50 b at a longitudinalmidportion thereof. Each of the mirrors 50 a, 50 b further includes alower biasing spring 104 a (only one shown) biasing a respective mirror50 a, 50 b at a longitudinal portion thereof toward its respectiveadjusting screw 100 a. Each of the mirrors 50 a, 50 b is adjustable byits respective adjusting screw 100 a (only one shown) wherein movementof the screw 100 a into or out of the lower housing portion 60 causespivotal movement of the mirror 50 a, 50 b about its respective pivotmounts 96 a, 96 b whereby angular adjustment of the mirror 50 a, 50 b isprovided.

[0053] Referring to FIGS. 5 and 6, an adjustable mounting for eachf-theta lens 52 a, 52 b will be described with particular reference tothe f-theta lens 52 a. The lower housing portion 60 is provided withfirst and second lens housings 106 a, 106 b for receiving the f-thetalenses 52 a, 52 b (only one shown in FIG. 6). As may be seen withreference to the lens housing 106 b, a plurality of support surfaces 108b are provided for supporting the lens 50 b, wherein substantiallysimilar support surfaces are provided on the lens housing 106 a forsupporting the lens 52 a. In addition, as illustrated with reference tolens housing 106 a, a pair of resilient mounting clips 110 a areprovided at either end of the lens 52 a for biasing the lens 52 a towardits support surfaces, wherein it should be understood that a similarpair of clips are provided for biasing the lens 52 b onto itscorresponding support surfaces 108 b.

[0054] Pairs of adjusting screws 112 a, 112 b, see also FIG. 2, (alsoreferred to herein as an “adjustment mechanism”) are provided extendinginto the respective lens housings 106 a, 106 b for engagement withlongitudinally spaced side portions of the lenses 52 a, 52 b whereby thelenses 52 a, 52 b may be adjusted in a process direction extendingperpendicular to the parallel sides 88, 90 of the lower housing portion60. In addition, each lens 52 a, 52 b is biased by a pair of springclips 114 a (only one pair shown) for movement toward its respectiveadjusting screws 112 a, 112 b.

[0055] Referring to FIGS. 2-4, the upper housing portion 58 fits intothe lower housing portion 60, and in particular, the lower housingportion 60 includes supporting reference surfaces 118, 120, 122 forengaging the second side 68 of the upper housing portion 58 to therebysupport the upper housing portion 58 at a predetermined verticallocation within the lower housing portion 60. In addition, the lowerhousing portion 60 defines alignment pins 124, 126 wherein the alignmentpin 124 engages a hole 128 (FIG. 3) in the upper housing portion 58, andthe alignment pin 126 engages a slot 130 in the upper housing portion58. Thus, the engagement of the pins 124, 126 with the hole 128 and slot130 operate to align the upper housing portion 58 in a particularpredetermined horizontal position relative to the lower housing portion60. The upper housing portion 58 is held in position on the lowerhousing portion 60 by screws (not shown) passing through holes 132, 134,136 in the upper housing portion 58 and threadably engaging within holes138, 140, 142 in the lower housing portion 60.

[0056] It should be understood that with the upper and lower housingportions 58, 60 assembled, the optical components of the first andsecond post-scan optical assemblies 16, 18 defining the correspondingoptical paths are located in a compact, folded configuration. Inparticular, with the two housing portions 58, 60 assembled, the secondfold mirrors 48 a, 48 b are located in a position generally adjacent tothe plate 82 of the lower housing portion 60, aligned with the thirdfold mirrors 50 a, 50 b, and the first fold mirrors 44 a, 44 b directthe scanning beams for the two optical paths of the first and secondpost-scan optical assemblies 16, 18 inwardly toward each other toreflect off of the second fold mirrors 48 a, 48 b generally locatedbeneath the scanning assembly 14. Accordingly, the space beneath and oneither side of the scanning assembly 14 is efficiently used which, incombination with using a common scanning assembly 14 for generatingscanning beams for both post-scan assemblies 16,18, facilitatesminimization of the space requirements for the present laser scanningunit.

[0057] Further, it should be noted that the present laser scanning unit7 is intended for use in a color laser printer using two of thedisclosed laser scanning units 7 for generating latent images which,once developed on corresponding PC drums, may form black, magenta, cyanand yellow toner images, such that each of the laser scanning units 7 ina printer is provided to generate two different latent images. In orderto provide for accurate registration between toner images, accuratealignment is required between the two scan lines 54 a and 54 b producedby each laser scanning unit 7, including alignment of the distancebetween the two scan lines in the process direction, perpendicular tothe direction of the scan lines 54 a and 54 b, and also includingadjustment of the amount of bow or bow error of each of the two scanlines 54 a and 54 b, such that variance or any difference between thebow of the two scan lines 54 a and 54 b is minimized. “Bow error”typically is arc-shaped and is generally measured in the processdirection from a desired straight line extending in the scan directionbetween the start and end points of the scan line to the maximum offseton the arc-shaped scan line.

[0058] The location of each scan line 54 a and 54 b in the processdirection is adjusted so as to be within a predetermined tolerance,e.g., plus or minus 0.25 mm, of a desired position for that scan line.The desired process position for each scan line 54 a, 54 b may berelative to the same datum point (not shown) or separate datum points(not shown) provided on the housing 56. It is also contemplated that onescan line, either scan line 54 a or scan line 54 b, may be adjustedrelative to its corresponding datum point and, thereafter, the desiredprocess position for the remaining scan line may be adjusted so as totake into account any offset in the process position of the one scanline from its desired process position, e.g., the one scan line may beoffset 0.24 mm from its desired process position yet still be within theallowable tolerance. Hence, by making separate adjustments for each scanline 54 a and 54 b, the variation in distance d in the process directionbetween the scan lines 54 a, 54 b, see FIG. 1, may be held to within apreselected tolerance, e.g., plus or minus 0.25 mm, in order to maintainacceptable image registration between the different color toner images.For example, in a preferred embodiment, the distance d between the scanlines 54 a, 54 b is 101 mm plus or minus 0.25 mm.

[0059] The configuration of the present laser scanning unit 7facilitates formation of the scan lines 54 a, 54 b such that the bowerror (curved distortion away from a desired straight scan line) andlinearity error (PEL spacing variation along a scan line from a desiredconstant spacing between PELs) of each scan line 54 a, 54 b has asimilar shape and magnitude in order to ensure that no point along thetoner image of one color is misregistered relative to another colorimage by more than a predetermined amount. One aspect for achievingproper registration, such that the linearity error for the first scanline 54 a is substantially the same in shape and magnitude to that ofthe second scan line 54 b, is to ensure that the beams produced by thefirst and second laser diode/pre-scan optical assemblies 11, 13 aredirected at angles α₁ and α₂ relative to lines L₁ and L₂ extendingsubstantially parallel to the scan lines 54 a, 54 b, and wherein theangles α₁ and α₂ have substantially the same magnitude but extend inopposite directions, i.e., are mirror images of one another. Hence, theangles α₁ and α₂ are symmetrical about a plane P_(L) passing through thecenter of the scanner 14 and substantially parallel to the scan lines 54a, 54 b, see FIG. 2. In addition, bow error for each scan line 54 a and54 b is adjusted to within a desired tolerance of a nominal bow value,e.g., plus or minus 20 microns from a nominal value which may range from0 to several hundred microns of bow. It is noted that the actual valueof bow is not important (as long as it does not exceed several hundredmicrons). However, the variance in the bow values of the first andsecond scan lines 54 a and 54 b must be small, such as ±20 microns.

[0060] Adjustment of bow and process position for each of the first andsecond scan lines 54 a and 54 b is effected using an alignment processincluding an alignment of the third fold mirrors 50 a, 50 b and thesecond f-theta lenses 52 a, 52 b, which will now be described.

[0061] In adjusting the bow value for and setting the proper alignmentof each scan line 54 a, 54 b, the second f-theta lenses 52 a, 52 b areinitially placed at a reference location within the lens housings 106 a,106 b. As may be seen in FIG. 6 with reference to lens housing 106 b,alignment tabs 146 b are molded into the lens housing 106 b and definean alignment edge 148 b. During initial set up of the scanning unit, thesecond f-theta lens 52 b is aligned, using adjustment screws 112 b, suchthat a longitudinal edge of the lens 52 b is in alignment with thealignment edge 148 b of each alignment tab 146 b. The lens housing 106 aincludes similar alignment tabs defining alignment edges for use inaligning the second f-theta lens 52 a to an initial position.

[0062] For purposes of illustration, the bow value and process positionof the first scan line 54 a is adjusted before that of the second scanline 54 b. However, the process can be performed in a reverse order orsimultaneously.

[0063] Referring to FIG. 7, a calibration arrangement for use inadjusting bow and process position of the scan line 54 a isschematically illustrated, it being understood that a similararrangement is also provided for the scan line 54 b. The calibrationarrangement includes first and second end camera units 150 a, 152 a,each comprising a single camera, and a central camera unit 154 acomprising first and second cameras 192 a, 192 b and a beam splitter 184a. The beam splitter 184 a splits the incoming beam into first andsecond beam portions 188 a, 186 a which are received by the first andsecond cameras 192 a, 192 b, respectively. It is noted that the centralcamera unit 154 a may only comprise a single camera. In point of fact,only one of the first and second cameras 192 a, 192 b is used during bowand process position adjustment.

[0064] The camera units 150 a, 152 a, 154 a are positioned at locationswhich correspond to the latent image-receiving plane defined by theouter surface of the photoconductive drum 20, wherein the end cameraunits 150 a, 152 a are located at positions corresponding to the ends ofthe scan line 54 a, and the central camera unit 154 a is located suchthat the scanning line intersects the beam splitter 184 a. The cameraunits 150 a, 152 a, 154 a are connected to a processor P_(R) forprocessing signals from the camera units 150 a, 152 a, 154 acorresponding to the particular location on each camera unit traversedby the scanning beam. It should be understood that information from theend camera units 150 a, 152 a is used to determine the process positionand the skew orientation of the scan line 54 a, and information from theend camera units 150 a, 152 a in combination with information from thecentral camera unit 154 a is used to determine the bow of the scan line54 a. In addition, it should be noted that the calibration arrangementillustrated in FIG. 7, as well as the corresponding calibrationarrangement (not shown) for aligning the scan line 54 b, is preferablymounted in an alignment fixture (not shown) to which the scan unit 7 maybe mounted.

[0065] It is noted that pivotable movement of the third fold mirror 50 aeffects a change in the bow value for the scan line 54 a as well as achange in the position of the scan line 54 a in the process direction.Furthermore, linear movement of the second f-theta lens 52 a in theprocess direction also effects a change in the scan line bow value andscan line position in the process direction. In order to minimize thenumber of third fold mirror 50 a and second f-theta lens 52 a adjustmentsteps required to adjust bow and process position of the scan line 54 aduring manufacturing, the present invention provides for making only twoadjustments after the third fold mirror 50 a has been rotated such thatthe scanning beam has been sensed by the camera units 150 a, 152 a and154 a as well as by a corresponding horizontal sync (Hsync) sensor (notshown) integral to the laser scanning unit 7: an adjustment to the thirdfold mirror 50 a and a final adjustment to the second f-theta lens 52 a.The Hsync sensor senses the beam just prior to the beam writing orimaging a line of print elements (PELs) or dots on the PC drum 20. Morespecifically, the invention provides for adjusting the third fold mirror50 a such that the bow value for the scan line 54 a is equal to a bowadjustment target value B_(T), which typically is not equal to a final,desired nominal bow value. The bow adjustment target value B_(T) isdetermined such that adjustment of the second f-theta lens 52 a toposition the scan line 54 a in the process direction within apredetermined tolerance of a desired process position also results inthe bow value for the scan line 54 a falling within a desired toleranceof a nominal bow value, e.g., 0 microns of bow ±20 microns.Alternatively, if the second f-theta lens 52 a is adjusted such that thescan line bow value falls within a desired tolerance of a nominal bowvalue, e.g., 0 mm of bow ±20 microns, the scan line 54 a is also locatedin the process direction within a predetermined tolerance of a desiredprocess position.

[0066] It is also contemplated that the third fold mirror 50 a may berotated to a nominal position and, thereafter, the second f-theta lens52 a may be moved, i.e., translated, such that the bow value for thefirst scan line 52 a is equal to a first predetermined target value.Then, the third fold mirror 50 a may be rotated again such that the bowvalue for the scan line 52 a falls within a desired tolerance of anominal bow value, e.g., 0 mm of bow ±20 microns, or the scan line 52 ais located in the process direction within a predetermined tolerance ofa desired process position. When the third fold mirror 50 a is rotatedsuch that the scan line 52 a falls within a desired tolerance of anominal bow value, the position of the scan line 52 a should also belocated within a predetermined tolerance of a desired process position.Likewise, when the third fold mirror 50 a is rotated such that the scanline 52 a is located within a predetermined tolerance of a desiredprocess position, the bow value of the scan line 52 a should fall withina desired tolerance of a nominal bow value.

[0067] With the second f-theta lens 52 a positioned at the initialreference location, the third fold mirror 50 a is first pivoted to afirst location by means of the adjusting screw 102 a, see FIG. 4, suchthat the scanning beam 53 a is sensed by the Hsync sensor and the cameraunits 150 a, 152 a and 154 a. With the third fold mirror 50 a in itsfirst location, the processor P_(R) coupled to the camera units 150 a,152 a and 154 a determines an initial process position PE₁ and aninitial bow value BE₁ for the scan line 54 a. The initial processposition PE₁ may be determined using camera unit 150 a and camera unit152 a. The position where the beam 53 a strikes each camera unit 150 a,152 a is sensed by each camera unit 150 a, 152 a and provided to theprocessor P_(R), which determines the current scan line positionrelative to a datum point on the housing 56. The processor P_(R) furtherdetermines an initial bow value BE₁ by taking information from thecamera units 150 a, 152 a and 154 a regarding where the beam 53 acrosses the camera units.

[0068] Using the following equation and the initial process position PE₁and the initial bow value BE₁, the processor P_(R) determines the bowadjustment target value B_(T):

B _(T)=(R ₁ *S1)((PE ₁ −P ₂)+((R ₂ /R ₁)*B ₂)−((S ₂ /S ₁)*BE ₁))/((S ₁*R ₂)−(S ₂ *R ₁))

[0069] where:

[0070] PE₁=the initial process position;

[0071] BE₁=and the initial bow value;

[0072] P₂=the desired nominal process position;

[0073] B₂=the desired nominal bow value, e.g., 0 mm, which is determinedby taking an average of the bow of each of a predetermined number ofactual scanning units 7, e.g., 50 units, after those units have beencalibrated or adjusted to an acceptable bow value;

[0074] R₁=−0.0707 mm/mm of translation—A first linearized sensitivity ofbow to translation of the second f-theta lens 52 a;

[0075] S₁=−0.304 mm/degree of rotation—A first linearized sensitivity ofbow to rotation of the third mirror 50 a;

[0076] R₂=1.501 mm/mm of translation—A second linearized sensitivity ofprocess position to translation of the second f-theta lens 52 a; and

[0077] S₂=2.933 mm/degree of rotation—A second linearized sensitivity ofprocess position to rotation of the third mirror 50 a.

[0078] The third fold mirror 50 a is then further rotated until themeasured bow value, as determined by the processor P_(R) from inputsfrom the camera units 150 a, 152 a and 154 a, is equal to the bowadjustment target value B_(T) or to within a predetermined tolerance ofthe target value B_(T). This is the first of the two adjustments notedabove.

[0079] Thereafter, the second f-theta lens 52 a is translated in theprocess direction via screws 112 a so as to set the final processposition for the scan line 54 a to within a predetermined tolerance of adesired process position relative to a datum point on the housing 56.This is the second of the two adjustments noted above. As further notedabove, adjustment of the second f-theta lens 52 a such that the scanline 54 a is positioned in a desired location in the process directionalso results in the bow value for the scan line 54 a falling within adesired tolerance of a nominal bow value, e.g., 0 microns of bow ±20microns.

[0080] The adjusting screws 112 a may also be used to adjust skew, asneeded, by moving one of the adjusting screws 112 a more than the otheradjusting screw 112 a in order to pivot the second f-theta lens 52 arelative to the process direction. It is believed that adjustment of thesecond f-theta lens 52 a to correct skew will have a small effect on bowand process position of the scan line 54 a. However, if the effect isfound to be unacceptable, then bow and process position of the scan line54 a can be adjusted one or more additional times after skew iscorrected.

[0081] It should be understood that the third fold mirror 50 b andsecond f-theta lens 52 b of the second post-scan assembly 18 may beadjusted in a manner similar to that described for the third fold mirror50 a and second f-theta lens 52 a of the first post-scan assembly 16 asrequired to obtain a desired process position for the scan line 54 arelative to a datum point on the housing 56, and a bow value fallingwith a desired tolerance of a nominal bow value, e.g., 0 microns of bow±20 microns. It is also noted that when adjusting the process positionof the second scan line 54 b, the desired process position may be variedso as to take into account any offset of the final position of the firstscan line 54 a in the process direction from a desired final processposition.

[0082] The two-step adjustment process for adjusting scan line bow andprocess position and the equation for bow adjustment target value B_(T)are applicable for use in adjusting the third fold mirrors 50 a, 50 band second f-theta lenses 52 a, 52 b of the first and second post-scanassemblies 16, 18 illustrated in FIG. 1 and 7A. Locations of specificpoints 1-8 along the optical path defined by the components of eachpost-scan assembly 16, 18 relative to a reflection point RP on thepolygon mirror 36, as illustrated in FIG. 7A, are as follows:

[0083] Point 1=−29.00 mm (X-direction); 0 mm (Z-direction);

[0084] Point 2=−20.96 mm (X-direction); −10.5 mm (Z-direction);

[0085] Point 3=−15.18 mm (X-direction); −18.04 mm (Z-direction);

[0086] Point 4=−3.81 mm (X-direction); −32.89 mm (Z-direction);

[0087] Point 5=−30.39 mm (X-direction); −32.89 mm (Z-direction);

[0088] Point 6=−29.08 mm (X-direction); −62.86 mm (Z-direction);

[0089] Point 7=−28.71 mm (X-direction); −71.35 mm (Z-direction);

[0090] Point 8=−23.85 mm (X-direction); −182.74 mm (Z-direction).

[0091] It is also noted that each first f-theta lens 46 a, 46 b has afirst surface 140 a, 140 b, see FIGS. 1, 7A-7C, having a radius R_(P1A),R_(P1B) in the process direction of 1034.05 mm and a radius R_(S1A),R_(S1B) in the scan direction of 1034.05 mm. Each first f-theta lens 46a, 46 b further includes a second surface 142 a, 142 b having a radiusR_(P2A), R_(P2B) in the process direction of 40.00 mm and a radiusR_(S2A), R_(S2B) in the scan direction defined by the followingequation:X=−(4.923×10⁻³y²)−(7.126×10⁻⁸y⁴)+(2.488×10⁻¹¹y⁶)−(2.632×10⁻¹⁴y⁸)+(4.599×10⁻¹⁸y¹⁰)where y extends in the scan direction and X is the sag value and extendsalong the system optical axis, see FIGS. 7A-7C.

[0092] Each second f-theta lens 52 a, 52 b has a first surface 150 a,150 b having a radius R_(P1A), R_(P1B) in the process direction definedby the following equation:

R=36.042−(8.889×10⁻⁴ y ²)+(2.370×10⁻⁷ y ⁴)+(8.235×10⁻¹² y⁶)−(3.358×10⁻¹⁵ y ⁸)+(1.050×10⁻¹⁹ y ¹⁰)+(1.032×10⁻²³ y ¹²)

[0093] where y extends in the scan direction and R is the radius value,R_(P1A), R_(P1B), and a radius R_(S1A), R_(S1B) in the scan direction of235.00 mm. Each second f-theta lens 52 a, 52 b further includes a secondsurface 152 a, 152 b having a radius R_(P2A), R_(P2B) in the processdirection of 18.75 mm and a radius R_(S2A), R_(S2B) in the scandirection of 386.82 mm.

[0094] The equation for bow adjustment target value B_(T) was derived asfollows.

[0095] Initially, the following sensitivity equations were provided:

(R ₁)*(Z)+(S ₁)*(Θ)=B  (Equation 1)

(R ₂)*(Z)+(S ₂)*(Θ)=P  (Equation 2)

[0096] where

[0097] Z=amount of translation of the second f-theta lens (mm);

[0098] Θ=amount of rotation of the third fold mirror (degrees);

[0099] R₁=A first linearized sensitivity of bow to translation of thesecond f-theta lens, determined either experimentally or using aconventional optical system model, both of which are commonly known tothose skilled in the art;

[0100] S₁=A first linearized sensitivity of bow to rotation of the thirdmirror, determined either experimentally or using a conventional opticalsystem model, both of which are commonly known to those skilled in theart;

[0101] R₂=A second linearized sensitivity of process position totranslation of the second f-theta lens, determined either experimentallyor using a conventional optical system model, both of which are commonlyknown to those skilled in the art;

[0102] S₂=A second linearized sensitivity of process position torotation of the third mirror, determined either experimentally or usinga conventional optical system model, both of which are commonly known tothose skilled in the art;

[0103] B=change in bow value; and

[0104] P=change in process position of a scan line.

[0105] It was assumed that only the third fold mirror is first adjustedto change its position Θ₁ (degrees) to yield a bow change B₁ equal tothe amount of bow change needed to achieve a desired, intermediate bowtarget value, B_(T). Thus, Z=0 is input into Equation 1 above to yield:

(S ₁)*(Θ₁)=B ₁  (Equation 3)

[0106] The amount of bow change B₁ needed in this first step is a linearcombination of the change needed to eliminate the initial bow value BE₁and the amount of bow change needed to achieve a bow adjustment targetvalue B_(T).

B ₁=(−BE ₁)+B _(T)  (Equation 4)

[0107] With Z=0 and the third fold mirror rotated Θ₁ to achieve thedesired bow adjustment target value B_(T), a new process position P₁comprises an initial process position PE₁ plus the change in processposition resulting from rotation of the third fold mirror, as determinedfrom Equation 2 with Z=0:

P ₁ =PE ₁+(S ₂)*(Θ₁)  (Equation 5)

[0108] It is desired to leave the third fold mirror in the positionresulting from rotation amount Θ₁ and translate the second f-theta lensby an amount Z₁ such that the process position P₁ is changed to thedesired final position, P₂, and the bow value becomes B₂. Hence, thechange in rotation of the third mirror is Θ=0 and Equation 2 yields:

(R ₂)*(Z ₁)=−P ₁ +P ₂  (Equation 6)

[0109] and the resulting process location becomes P₂.

[0110] The new bow value, B₂, is a linear combination of the bow valuefrom the third fold mirror rotation, B_(T), and the bow change caused bythe second f-theta lens translation, Z₁:

B ₂ =B _(T)+(R ₁)*(Z ₁)  (Equation 7)

[0111] A unique solution exists for Equations 3-7, which can bedetermined numerically using conventional matrix solver software such asExcel or Mathcad (or can be solved in closed form to yield the equationset out above for the bow adjustment target value B_(T)). In matrixform, the five unknowns are: Θ₁; Z₁; B_(T); B₁; and P₁. The inputvariables are BE₁, (measured); PE₁, (measured); P₂ (the desired finalprocess location value) and B₂ (the desired nominal bow value, e.g., 0mm, which is determined by taking an average of the bow of each of apredetermined number of actual scanning units 7, e.g., 50 units, afterthose units have been calibrated or adjusted to an acceptable bowvalue).

[0112] Proper alignment of the scan lines 54 a, 54 b further requiresproper alignment of the laser diode/pre-scan optical assemblies 11, 13in order to provide proper location of the beams reflected from thescanning assembly 14 to the post-scan assemblies 16, 18, as provided byadjustment of the collimation assemblies 27 a, 27 b, and to provide adesired spot size for forming the scan lines 54 a, 54 b at thephotoconductive drums 20, 22, as provided by adjustment of the pre-scanlens carriers 34 a, 34 b.

[0113] Referring to FIG. 8, the collimation assemblies 27 a, 27 b areeach aligned to a desired direction using a quad cell sensor 160supported in a mount structure 162 which comprises an alignment mount164 extending through an opening 166 in the upper housing portion 58. Atthis juncture, the lower housing portion 60 is not coupled to the upperhousing portion 58. The quad cell sensor 160 is positioned at a point ofintersection between the two beams produced by the first and secondlaser diode/pre-scan optical assemblies 11, 13, and the alignment of thecollimation assemblies 27 a, 27 b is performed prior to installation ofthe scanning assembly 14. The quad cell sensor 160 detects the positionof each beam in scan and process directions. Each collimation assembly27 a, 27 b is correctly aligned when its corresponding beam impingesupon a predetermined position of the quad cell sensor 160, e.g., acenter portion thereof. Hence, alignment of the collimation assemblies27 a, 27 b occurs prior to adjustment of bow and process position forthe scan lines 54 a, 54 b.

[0114] The alignment of the collimation assemblies 27 a, 27 b will bedescribed with particular reference to the first collimation assembly 27a, an enlarged view of which is shown in FIG. 9. The collimationassembly 27 a includes an L-shaped base member 168 supported on afixture structure comprising at least support members 170, 172, andincluding a pivot pin 174 wherein the fixture structure may be formed asan integral part of the upper housing portion 58 or may be formed as aseparate structure fit to the upper housing portion 58, such asincluding members extending upwardly through the upper housing portion58 to engage and/or support the base member 168. The collimation housing30 a is supported on the base member 168, and together define acollimation structure. The base member 168 is movable relative to thefixture structure 170, 172, 174 to position the beam output from thecollimation assembly 27 a in first and second directions which aresubstantially perpendicular to one another. In particular, the fixturestructure pin 174 engages an aperture in the base member 168 to permitpivotal movement of the base member 168 in a horizontal plane about anaxis defined by the pin 174. Further, fasteners 176, 178 extend throughthe base member 168 into threaded engagement with the fixture elements170, 172 wherein apertures in the base member 168 accommodating thefasteners 176, 178 are oversized in order to permit pivotal movement ofthe base member 168 about the pin 174.

[0115] In addition, a compressible spring structure is provided betweenthe base member 168 and the fixture structure element 170, and in thepreferred embodiment, the compressible structure comprises a bellevillewasher 180. Compressive force applied by the fastener 176 to move a leg177 of the base member 168 downwardly toward the fixture element 170causes pivotal movement of the base member 168 in a vertical directionwhereby the vertical position of the laser beam originating from thecollimation assembly 24 a is adjusted in a vertical direction, i.e., inthe process direction. The belleville washer 180 defines a compressiblespring biased spacer maintaining the base member 168 in contact with ahead portion of the fastener 176 while permitting vertical movement ofthe leg 177 relative to the fixture element 170. It is also contemplatedthat an element, other than a belleville washer, such as a conventionaljack screw (not shown), may be provided to adjust or reposition the basemember 168 relative to the upper housing portion 58. Accordingly,adjustment of the base member 168 provides both horizontal and verticaladjustment of the laser beam output from the collimation assembly 27 aand sensed by the quad cell sensor 160. Further, the collimationassembly 27 b is supported in substantially the same manner as thecollimation assembly 27 a for adjustment of the beam output from thecollimation assembly 27 b in a horizontal and vertical direction to besensed by the quad cell sensor 160. It is noted that movement of thecollimation assemblies 27 a and 27 b also results in movement of thecorresponding laser driver cards 28 a, 28 b.

[0116] As noted above, the positions of the pre-scan lens carriers 34 a,34 b are adjusted to provide the desired spot size at thephotoconductive drums 20, 22, and in particular, it is desirable toadjust the positions of the pre-scan lens carriers 34 a, 34 b such thatthe waist of each respective beam occurs at or near the point ofincidence on the corresponding PC drum 20, 22. In order to determine thelocation of the beam waist, a beam waist calibration arrangement isprovided for measuring the spot size at two spaced locations from whichthe waist location may be extrapolated in the following manner. Theadjustment of the pre-scan lens carriers 34 a, 34 b may be effectedprior to or after adjustment of bow and process position for the scanlines 54 a, 54 b. Preferably, the adjustment of the carriers 34 a, 34 boccurs after the scanning unit 7 has been fully assembly and adjustmentof bow and process position for the scan lines 54 a, 54 b has takenplace.

[0117] Initially, an average focus point f of a parabola plotted forlaser beam spot size is determined for a given pre-scan lens, which lensis used as pre-scan lenses 32 a, 32 b in the scanning unit 7. Thisinvolves moving the position of the pre-scan lens in small increments,e.g., 500 microns, relative to a nominal position of the lens from aknown location where a surface of a corresponding PC drum will bepositioned below a pre-scan lens 32 a, 32 b once the scanning unit 7 isassembled in a printer. At each pre-scan lens position, spot size of thebeam is measured using a conventional spot size sensor/camera at about15 beam locations spaced from the known PC drum surface location,including locations above the drum surface and below the drum surface.From the 15 spot size values, a curve, which will have the shape of aparabola, is generated as well as a parabolic equation for that curve orspot size data. Hence, an equation in the form of y=ax²+bx+c isdetermined. The parabola focus point f is then determined from theequation f=1/(4a), where “a” is taken from the determined parabolicequation. A plurality of “f” values are determined for a plurality ofpre-scan lens positions and an average f value is calculated.

[0118] The following equations A and B are given:

y _(c1)=(1/(4f))(x _(c1) ²)+bx _(c1) +c  (A)

y _(c2)=(1/(4f))(x _(c2) ²)+bx _(c2) +c  (B)

[0119] The two equations are solved for b and c as follows:

b=(−¼)[(−4*y _(c1) *f+x _(c1) ²+4*y _(c2) *f−x _(c2) ²)/(f*(x _(c1) −x_(c2)))]

c=(¼)[(4*y _(c2) *f*x _(c1) −x _(c2) ² *x _(c1)−4*x _(c2) *y _(c1) *f+x_(c2) *x _(c1) ²)/(f*(x _(c1) −x _(c2)))]

[0120] where:

[0121] x_(c1)=optical distance of a first spot size sensor/camera 192 a,see FIG. 10, from a known location of a surface of a corresponding PCdrum to be positioned below the scanning unit 7 in a printer;

[0122] x_(c2)=optical distance of a second spot size sensor/camera 192 bfrom a known location of a surface of a corresponding PC drum to bepositioned below the scanning unit 7 in a printer;

[0123] y_(c1)=spot size as sensed by the first spot size sensor 192 a;and

[0124] y_(c2)=spot size as sensed by the second spot size sensor 192 b.

[0125] For a given position of a pre-scan lens 32 a, 32 b in the housing56, a waist position of the laser beam passing through the lens relativeto the known PC drum surface position, i.e., the distance the beam waistis away from the known PC drum surface position, can be determined bytaking the derivative of the parabolic equation and solving for zero.This provides:

X=−b/(2a)

[0126] where X=the location of the beam waist relative to the knownlocation of the surface of a corresponding PC drum to be positioned in aprinter below the scanning unit 7;

a=1/(4f)

[0127] where f is the average parabola focus point calculated as notedabove.

[0128] Hence, an operator can reposition a pre-scan lens 32 a, 32 b sothat the waist of the corresponding laser beam is positioned at or neara known location of a surface of a PC drum to be positioned below thescanning unit 7. This involves taking data from two spot sizesensors/cameras 192 a, 192 b, see FIG. 10, located a known opticaldistance from a known location of a surface of a PC drum to bepositioned below the scanning unit 7 and solving for b and then X. Aconventional microprocessor, such as processor P_(R) illustrated in FIG.7, could be used to solve for b and X. Once, X is at or near zero, thebeam waist is at or near the know PC drum surface position. It is alsocontemplated that a look-up table or linear equation can be generated toprovide an operator with information regarding a distance and directionthe pre-scan lens 32 a, 32 b must be moved from the measured position sothat the beam waist is at or near the known location of a surface of aPC drum to be positioned below the scanning unit 7. It is furthercontemplated that, after a pre-scan lens has been repositioned, new spotsize values can be received from the sensors 192 a, 192 b and used todetermine if the beam waist is at or near the known location of asurface of a PC drum to be positioned below the scanning unit. Visualindicators, such as green and red lamps, indicating respectively thatthe pre-scan lens is properly positioned or not, may be provided.

[0129] Referring to FIG. 10, a pre-scan lens calibration arrangement 182a is schematically illustrated positioned below the first post-scanoptical assembly 16. The calibration arrangement 182 a may comprise thecamera unit 154 a illustrated in FIG. 7. The calibration arrangement 182a includes a beam splitter 184 a positioned to receive the beam from thesecond f-theta lens 52 a, which beam is split into first and second beamportions 186 a, 188 a. First and second cameras 192 a, 192 b arepositioned to receive the first and second beam portions 188 a, 186 a,respectively, and measure the spot size for each of the first and secondbeam portions 186 a, 188 a. The first camera 192 a is positioned apredetermined optical distance x_(c1) above or in front of a knownlocation of a surface of a corresponding PC drum to be positioned belowthe scanning unit 7. The second camera 192 b is positioned apredetermined optical distance x_(c2) below or beneath a known locationof a surface of a corresponding PC drum to be positioned below thescanning unit 7. During testing, the scanning 7 unit is not mountedwithin a printer housing. Hence, a PC drum is not positioned below theunit 7 during calibration of the position of the pre-scan lens 32 a.From the two measured spot sizes, y_(c1) and y_(c2), and the knownoptical distances x_(c1) and x_(c2) from the cameras 192 a and 192 b toa known PC drum surface location, it is possible to calculate thelocation X of the beam waist using the equations noted above whereinmovement of the pre-scan lens carrier 34 a longitudinally along apre-scan lens support structure 194 a operates to adjust the position ofthe beam waist to the desired location. It should be understood that asimilar calibration assembly is provided for positioning of the pre-scanlens carrier 34 b.

[0130] The pre-scan lens support 194 a includes guide rail portions 196a supporting the pre-scan lens carrier 34 a and underlying the pre-scanlens 32 a. The pre-scan lens carrier 34 a includes a pair of spacedengagement portions 198 a, 200 a for engaging a guide bar 202 a of thesupport structure 194 a to facilitate locating the pre-scan lens carrier34 a in a direction transverse to its longitudinal movement. A tabportion 204 a of the pre-scan lens carrier 34 a engages under the guidebar 202 a to maintain one side of the pre-scan lens carrier 34 a inengagement with the guide rail portions 196 a, and an opposite side ofthe pre-scan lens carrier 34 a includes a tab 206 a defining a slot 208a through which is positioned a fastener 210 a for verticallypositioning the side of the pre-scan lens carrier 34 a opposite from theguide bar 202 a, and for locking the pre-scan lens carrier 34 a at itsadjusted location. A similar pre-scan lens calibration arrangement andsimilar support structure is provided for calibration of the secondpre-scan lens carrier 34 b whereby the second pre-scan lens carrier 34 bis adjustable in the same manner as described for the first pre-scanlens carrier 34 a.

[0131] It should be understood that by providing the above-describedadjustments for the collimation assemblies 27 a, 27 b and for thepre-scan lenses 32 a, 32 b, it is possible to provide the housing 56 forthe present laser scanning unit as a molded unit and in which thetolerance limits for the upper and lower housing portions 58, 60 may berelaxed, resulting in reduced molding costs for forming the housing 56.In particular, since the alignment of the beams formed at the laserdiode/pre-scan optical assemblies 11, 13 is adjustable subsequent toassembly within the upper housing portion 58, it is possible to form theupper housing portion 58 with less strict tolerance limits, such as maybe required if the alignment of the optical components relied entirelyon the location of the mounting points provided in the housing.

[0132] Similarly, the adjustment of the third fold mirrors 50 a, 50 band second f-theta lenses 52 a, 52 b provides for adjustment of the scanline characteristics whereby greater variations in tolerance andassembly location between the upper and lower housing portions 58, 60may be readily accommodated. Further, the adjustability of the thirdfold mirrors 50 a, 50 b and second f-theta lenses 52 a, 52 b, andresulting ability to construct the present laser scanning unit as a twopiece housing, enables the scanning unit to be formed in a compactconfiguration facilitating assembly by providing for mounting of opticalcomponents on both the upper housing portion 58 and lower housingportion 60 prior to assembly of the housing 56.

What is claimed is:
 1. A laser scanning unit comprising: a housing; ascanning assembly including a scanning device; a first light beam sourcedirecting a first light beam toward said scanning device; a second lightbeam source directing a second light beam toward said scanning device;and first and second post-scan optical assemblies located to receivefirst and second scanning beams reflected from said scanning device,each said post-scan optical assembly comprising a first, second andthird fold mirror, a first f-theta lens located between said first andsaid second fold mirrors, and a second f-theta lens located to receivelight reflected from said third fold mirror and output a compensatedscanning beam for generating a scan line on a correspondingphotoconductive member.
 2. The laser scanning unit as set forth in claim1, wherein light reflected from each of said second fold mirrors towarda respective one of said third fold mirrors lies in a plane which isgenerally parallel to a plane containing the light beam produced by arespective one of said first and second light beam sources.
 3. The laserscanning unit as set forth in claim 1, wherein at least one of saidfirst and second post-scan optical assemblies includes an adjustmentmechanism for adjusting its corresponding second f-theta lens whereby ascan line formed by a scanning beam passing through said correspondingsecond f-theta lens is adjusted.
 4. The laser scanning unit as set forthin claim 3, wherein said adjustment mechanism adjusts said correspondingsecond f-theta lens in linear movement in a process direction generallyperpendicular to the scan line.
 5. The laser scanning unit as set forthin claim 3, wherein said corresponding second f-theta lens includesopposing, first and second longitudinal ends and said adjustmentmechanism provides independent adjustment of said first and second endsto adjust one or more of a process direction position of the scan line,bow of the scan line, and skew orientation of the scan line.
 6. Thelaser scanning unit as set forth in claim 1, wherein said third foldmirror of at least one of said first and second post-scan opticalassemblies is rotatably adjustable about a longitudinal axis of saidthird fold mirror to adjust the bow and process direction position ofthe corresponding scan line.
 7. The laser scanning unit as set forth inclaim 1, wherein each of said first fold mirrors reflects a respectivescanning beam inwardly and downwardly and said second fold mirrors ofsaid first and second post-scan optical assemblies are locatedsubstantially below said scanning device for receiving the reflectedscanning beams passing through said first f-theta lenses from said firstfold mirrors.
 8. The laser scanning unit as set forth in claim 1,wherein said scanning device comprises a polygon mirror supported forrotation about a rotational axis and said first light source and saidsecond light source are located on the same side of a plane containingsaid rotational axis of said polygon mirror and extending perpendicularto the scan lines, and said first and second light sources are locatedat substantially similar mirror image angles to parallel lines which aresubstantially parallel to the scan lines.
 9. A laser scanning unitcomprising: a scanning assembly including a scanning device; at leastone light beam source directing a light beam toward said scanningdevice; at least one post-scan optical assembly including a plurality ofoptical components; an upper housing portion and a lower housing portionwherein said upper and lower housing portions are joined together toform a housing for supporting said scanning assembly, said light beamsource and said post-scan optical assembly; and wherein at least one ofsaid optical components is mounted to said upper housing portion and atleast one of said optical components is mounted to said lower housingportion.
 10. The laser scanning unit as set forth in claim 9, whereinsaid at least one optical component mounted to said lower housingportion is adjustable to position a scan line generated by a scanningbeam output by said post-scan optical assembly.
 11. The laser scanningunit as set forth in claim 9, wherein said post-scan optical assemblycomprises a first, second and third fold mirror, a first f-theta lenslocated between said first and said second fold mirrors, and a secondf-theta lens located to receive light reflected from said third foldmirror.
 12. The laser scanning unit as set forth in claim 11, whereinsaid first and second fold mirrors and said first f-theta lens aremounted to said upper housing portion, and said third fold mirror andsaid second f-theta lens are mounted to said lower housing portion. 13.The laser scanning unit as set forth in claim 12, wherein said upperhousing portion includes upper and lower sides and said first foldmirror is mounted to said upper side and said second fold mirror andsaid first f-theta lens are mounted to said lower side.
 14. The laserscanning unit as set forth in claim 9, comprising two light beam sourcesand first and second post-scan optical assemblies located to receivefirst and second scanning beams reflected from said scanning device,said post-scan optical assemblies each comprising a plurality of opticalcomponents and wherein at least one of said optical components of eachof said first and second post-scan assemblies is mounted to said upperhousing portion and at least one of said optical components of each ofsaid first and second post-scan assemblies is mounted to said lowerhousing portion.
 15. The laser scanning unit as set forth in claim 9,wherein said upper and lower housing portions include first and secondalignment structures, said first alignment structure comprising a pin inone of said housing portions engaging a hole in the other of saidhousing portions, and said second alignment structure comprising a pinin said one of said housing portions engaging an elongated slot in saidother of said housing portions whereby said upper and lower housingportions are aligned with each other.
 16. A method of calibrating alaser scanning unit comprising a housing, a first light beam source, ascanning assembly including a scanning device, and a first post-scanassembly, said first post-scan assembly including at least one foldmirror, and at least one f-theta lens located downstream of said atleast one fold mirror to receive light reflected by said fold mirror andoutput a compensated first scanning beam for generating a first scanline on a first photoconductive member, said first scan line extendingin a scan direction which is substantially perpendicular to a processdirection, said method comprising the steps of: installing said secondf-theta lens and said fold mirror in said housing and, thereafter,adjusting the position of said fold mirror and said second f-theta lensto move the position and correct bow of said first scan line.
 17. Themethod as set forth in claim 16, further including the step of movingsaid f-theta lens to adjust a skew orientation of the scan line relativeto the process direction.
 18. The method as set forth in claim 16,including a second light beam source and a second post-scan assembly forforming a second scan line spaced from the first scan line in theprocess direction.
 19. The method as set forth in claim 16, wherein saidstep of installing said second f-theta lens and said fold mirror in saidhousing comprises the step of installing said f-theta lens at a nominallocation and said step of adjusting the position of said fold mirror andsaid second f-theta lens to move the position and correct bow of saidfirst scan line comprises the steps of moving said fold mirror to changethe first scan line to obtain a first bow value within a predeterminedrange and, thereafter, moving said f-theta lens to move the first scanline in the process direction to obtain at least one of a second bowvalue and a desired process location for said first scan line.
 20. Themethod as set forth in claim 19, wherein said first post-scan assemblycomprises a first, second and third fold mirror, a first f-theta lenslocated between said first and second fold mirrors, and a second f-thetalens located to receive light reflected from said third fold mirror andoutputting said first scanning beam for generating said first scan line,said step of moving said fold mirror including moving said third foldmirror to change the first scan line to obtain a first bow value withina predetermined range and said step of moving said f-theta lensincluding moving said second f-theta lens to move said first scan linein the process direction to obtain a second bow value and desiredprocess location for said first scan line.
 21. The method as set forthin claim 19, wherein said step of moving said f-theta lens to move thefirst scan line in the process direction results in said scan line beingadjusted to a final, desired bow value and a final, desired processlocation.
 22. A laser scanning unit comprising: a scanning assemblyincluding a scanning device; a light beam source directing a light beamtoward said scanning device; a post-scan optical assembly including apivotal mirror and a movable f-theta lens, said f-theta lens beingpositioned to receive light reflected by said mirror and outputting acompensated scanning beam for generating a scan line on a correspondingphotoconductive member, said mirror and said f-theta lens beingadjustable to move the position and correct bow of the scan line. 23.The laser scanning unit as set forth in claim 22, wherein said f-thetalens is longitudinally movable to correct bow of the scan line and tomove the position of the scan line in a process direction perpendicularto a scan direction of the scanning beam and is pivotally movable tocorrect a skew orientation of the scan line.
 24. The laser scanning unitas set forth in claim 22, wherein said pivotal mirror defines a thirdmirror and said f-theta lens defines a second f-theta lens and saidpost-scan optical assembly further comprises a first and a second foldmirror, and a first f-theta lens located between said first and secondfold mirrors, and wherein said second f-theta lens is located to receivelight reflected from said third fold mirror and outputting saidcompensated scanning beam for generating said first scan line.
 25. Alaser scanning unit comprising: a scanning assembly including a scanningdevice; a light beam source directing a light beam toward said scanningdevice; and a post-scan optical assembly including an f-theta lens forreceiving a scanning beam reflected by said scanning device andoutputting a compensated scanning beam for generating a scan line on acorresponding photoconductive member, said f-theta lens being movable tocorrect a skew orientation of the scan line.
 26. The laser scanning unitas set forth in claim 25, wherein said f-theta lens is supported forpivotal movement to correct said skew orientation.
 27. The laserscanning unit as set forth in claim 25, wherein said f-theta lensdefines a second f-theta lens and said post-scan optical assemblyfurther comprises a first, second and third fold mirror, and a firstf-theta lens located between said first and second fold mirrors, andwherein said second f-theta lens is located to receive light reflectedfrom said third fold mirror and outputting said compensated scanningbeam for generating said first scan line.